Diesel engine technology has made good progress over the last two decades. In addition to having good fuel economy and durability, diesel engines have gained a good reputation for performance and low hydrocarbon and carbon monoxide emissions. However, diesel engines have presented engineers with the formidable challenge of reducing nitric oxides (NOx) and particulate matter.
Exhaust gas recirculation (EGR) has been used for more than three decades in internal combustion engines to reduce NOx through increasing the specific heat coefficient of intake charge, which lowers the combustion temperature and dilutes intake air to slow down combustion. Recirculation of exhaust gas is usually accomplished by routing a portion of the exhaust gas back to the intake manifold where it is inducted into the cylinders along with charge air.
So far, despite its advantages, the use of EGR has fallen short of achieving desired diesel engine emission limits. Engineers have resorted to auxiliary emission control devices (also known as aftertreatment devices) to help meet the emissions reduction challenge. Typically, these devices require elevated exhaust temperatures to operate in an efficient manner.